When is a Smartphone no longer smart enough to be called a Smartphone?

I needed an extra phone and recently came into a BlackBerry World Edition phone, which will work for the service I had. I did not have a user manual so I had to scour the Net to find one. Navigating on it was not like any phone I ever used, it has a node that acts like a trackball. Once I found the model number and was able to find the manual, it was thin I discovered when the phone was introduced eight years ago it was called a Smartphone. I have had really basic and cheap phones that had more features, but they were newer phones. That got me thinking how many years do a Smartphone stay a Smartphone? When the technology filters down to inexpensive basic phones does that make a Smartphone, just a phone or is there still something in it to keep it being a Smartphone no matter how old it gets?